The present invention is a power steering assembly for a hydraulic power steering system of motor vehicles according to portions of patent claim 1. Furthermore, the invention also relates to a method for a steering support of a hydraulic power steering system of motor vehicles according to the generic portion of patent claim 10.
Among other things, power steering assemblies for hydraulic power steering systems of vehicles comprise servo valves also known as rotary servo valves or rotary slide valves. They control the hydraulic pressure and thus the steering support depending on the steering torque applied by the driver. Most frequently, rotary slide valves are used in which an input shaft connected via a steering column with a steering wheel rotates relative to a valve portion (also referred to as control sleeve or sleeve), which is connected with the output shaft and, in rack-and-pinion steering systems, with a steering pinion (also referred to as pinion). A torque-dependent adjustment of the control element of the servo valve, and thus a torque-dependent valve characteristic, is realized through a torsion system between the input shaft and the control element.
In order to realize various further functions of a torque adjuster, for example a lane departure assistant, over- or understeering assistant, tactile feedback, variable steering support, for instance dependent on the vehicle speed or load, city mode, automatic parking, steering torque superposition, an adjustment of the position of the control element independent from the applied torque is desirable for the purpose of influencing the steering support characteristic of the servo valve.
Such a servo steering valve is described in the published patent application DE 10 2004 049 686 A1. The document discloses an adjusting mechanism for a hydraulic steering valve in which a ring gear of a planetary gear train is mounted non-rotatably and a second ring gear of a second planetary gear train is mounted rotatably. The second ring gear can be rotated to a limited extent through an actuator. The actuator is configured as an electrical actuating element, for example as an electric motor whose rotary movement is converted into a lifting movement. The lifting movement is transmitted onto the rotatable ring gear through a lever. The rotation of the rotatable ring gear causes a rotation of a control element of the servo valve. This permits an adjustment of the relative angle between the control element and an output shaft of the servo valve.
In order to realize the functions of a torque adjuster cited above it is necessary to be able to set very small relative adjustments, particularly relative angles of less than 1/10 of a degree. According to the prior art, a difference angle between the ring gears is transferred slightly amplified to a relative angle between the control element and the output shaft, due to the transmission ratio of the planetary gear train. Thus, the difference angle set on the rotatably mounted ring gear has to be considerably smaller than 1/10 of a degree. This requires a highly precise actuator which is expensive for such an application. An articulation of the rotatably mounted ring gear which is almost free from play is also required for the precise adjustment of the difference angle, which, apart from entailing a complicated mechanism, also has adverse effects on production costs. Moreover, a durable and low-maintenance seal between the actuator and the lever for the articulated connection of the ring gear can only be ensured with a lot of effort, because the lever does not execute a linear movement during the adjustment process, but moves along a curved trajectory.